Antenna system with dielectric horn structure interposed between the source and lens



Dec. 3, 1968 H. E. BARTLETT ET AL 3,414,903

ANTENNA SYSTEM WITH DIELECTRIC HORN STRUCTURE INTERPOSED BETWEEN THESOURCE AND LENS Filed March 10, 1965 IFIGJ.

REFLECTED RAY DUE To PRESENCE of euaoms STRUCTURE LENS PHASE CENTER ofLAUNCHER '5 FIG. 3

FIG.4 RESULTANT AMPLITUDE I DISTRIBUTION GUIDE BASE 0|AMETER-l ENERGYTHAT WOULD BE LOST IN SPILLOVER LOBES INVENTORS m ABSENCE of DIELECTRICGU|DE HOMER E.- BARTLETT 8i LEROY PIETSCH B Y Z ATTORNEYS LAUNCHERPATTERN ANTENNA SYSTEM WITH DIELECTRIC HORN STRUCTURE INTERPOSED BETWEENTHE SOURCE AND LENS Homer E. Bartlett, Melbourne, and Le Roy Pietsch,Palm Bay, Fla., assignors to Radiation Incorporated, Melbourne, F la., acorporation of Florida Filed Mar. 10, 1965, Ser. No. 438,582 7 Claims.(Cl. 343753) ABSTRACT OF THE DISCLOSURE An antenna system comprising asource of electromagnetic waves, a lens, and a dielectric horn structureinterposed between the'source and lens. The first null of the radiationpattern produced by the source is provided at an angle approximatelyequal to the sum of the guiding structure taper angle and the complementof the critical angle of the structure dielectric, the critical angle ofthe structure dielectric being the angle of incidence of internalelectromagnetic waves on the boundary of the structure above which totalreflection of the incident wave is achieved. Whereby all of the energyin the main lobe of the radiation pattern is confined interiorly of theguiding structure and directed toward the lens.

The present invention relates generally to antenna systems, and moreparticularly to high aperture efliciency dielectric antennas.

In prior art antenna systems which have been employed to produce ahighly directive radiation pattern, such as reflector antennas and lensantennas, numerous proposals have been made for improving apertureefiiciency to the maximum theoretical efliciency of 100 percent. In lensand reflector antennas, the problem'is aggravated by the lossof energyradiated from the feed or exciter because of spillover radiation; thatis, that portion of the radiated energy which fails to strike {the lensor reflector. While lens antennas are generallyjsuperior to reflectorantennas from a noise temperature standpoint in that the spillover lobesof the former are in a forward direction, nevertheless the directivitycharacteristics, and hence aperture efliciency, fall significantly belowthe maximum attainable values for aperture-type antennas. This is aresult of the radiation losses caused byfthe nonincidence of theradiated waves on the lens. Hence, efficiency and noise temperatureimprovements can be obtained by reducing the forward spillover.

In accordance with an embodiment of the present invention, an antennasystem is provided which comprises an electromagnetic wave transducer,such as an exciter, a dielectric guiding structure, and a lens. Thedielectric guiding structure is interposed between the wave transducerand lens, and is provided with an increasing taper. The dimensions ofthe source of radiation, or electromagnetic wave transducer, may bepredetermined to provide a first null of the radiation pattern at anangle approximately equal to the sum of the guiding structure taperangle and the complement of the critical angle of the structuredielectric. Portions of those waves which are radiated from thetransducer at angles between the taper angle and the sum of the taperangle and the critical angle complement, and would not otherwise beincident on the lens in the absence of the guiding structure, strike theboundary between the guiding structure and free space at an anglegreater than the critical angle of the boundary, and are totallyreflected to the lens, thus significantly reducing spillover. Inaddition to reduced spillover, the amplitude distribution across thebase becomes more ited States Patent nearly uniform with the attendantefficiency increase. The lens itself corrects the phase distributionexisting across the base of the guiding structure such that a constantphase distribution is provided thereby.

It is, accordingly, a primary object of the present invention to providean antenna system having high aperture efiiciency.

It is another object of the present invention to provide a lens antennahaving a dielectric guiding structure to reduce spillover.

It is still another object of the present invention to provide anaperture-type antenna having substantially uniform amplitudedistribution and constant phase distribution of electromagnetic energyacross the aperture.

The above and still further objects, features and attendant advantagesof the present invention will become apparent from a consideration ofthe following detailed description of specific embodiments thereof,especially when taken in conjunction with the accompanying drawings inwhich:

FIGURE 1 is a perspective view of an exemplary dielectric guidingstructure in accordance with the present invention;

FIGURE 2 is an exploded planar diagram of an antenna system employingthe dielectric guiding structure of FIGURE 1;

FIGURE 3 illustrates the preferred radiation distribution pattern of anexciter employed in the antenna system of FIGURE 2; and

FIGURE 4 illustrates the resultant amplitude distribution across thebase of the dielectric guiding structure.

Referring now to the drawings, FIGURE 1 illustrates an exemplarydielectric guiding structure which may be employed in antenna systemssuch as will hereinafter be described. In its exemplary form thedielectric medium 10 comprises a solid conical structure for guidingenergy radiated from an exciter in the direction of increasing diametertoward a lens (as shown in FIGURE 2). The specific structure of thedielectric guide will, of course, depend upon the shape and dimensionsof the associated antenna elements; that is, the electromagnetic wavetransducer (exciter or receiving element) and the lens. The guide mayreadily be constructed of any dielectric material, such as Styrofoam,cross-linked polystyrene (e.g. Rexolite 1422), or artificial dielectricmaterial capable of providing the desired critical angle for aparticular application.

The operation of the dielectric antenna system may be best understood byreference to FIGURE 2, wherein heuristic ray tracing theory is employedfor purposes of analysis, and wherein is shown, in planar diagrammaticform, an exploded view of the antenna system. An electromagnetictransducer 12, such as a horn, is suitably coupled to a waveguide 13 orother appropriate signal transmission line depending on the particulartransducer. A dielectric guiding structure 10 of the type shown in FIG-URE l is inserted into the mouth of horn 12, and a phase correcting lens15 is disposed adjacent the base of the guiding structure.

Exciter 12 provides a means for transferring electromagnetic energy fromthe input transmission line, such as waveguide 13, to dielectric guidingstructure 10 and may comprise, for example, a metal horn, as shown, adielectric rod or tube antenna, a helix, a log periodic array, or someother small radiating or receiving element.

The dielectric guiding structure is tapered or flared at an angle on theorder of the complement of the critical angle of the particulardielectric employed. As used here, the term critical angle has its usualdefinition; that is, the angle defining total internal reflection, orthe angle marking the dividing point between which a ray incident on theboundary between two dielectric media will either be totally reflectedtherefrom or totally or partially transmitted therethrough. Totalinternal reflection will occur for rays incident on the boundary at anyangle greater than the critical angle of the particular dielectricmaterial in which the ray is travelling.

The lens 15 is employed to modify the phase distribution of theelectromagnetic waves appearing across the base of the guiding structureto provide a constant phase distribution for waves emanating from thesystem. The lens may be of conventional design, such as a dielectriclens for delaying the electromagnetic wave in accordance with itsdielectric constant to convert a spherical wave front into a plane wavefront. The focal point 17 of lens 15 is positioned in the dielectricmedium of the guiding structure such that, when the antenna componentsare assembled, it is at the phase center of the exciter. In the eventthat the lens is fabricated of a dielectric material, its dielectricconstant may be selected to be greater than that of guide in order tominimize length and weight, but this is not critical for properoperation.

When the exciter dimensions are so chosen that the first null of theradiation pattern occurs at an angle approximately equal to the sum ofthe guiding structure taper angle and the complement of the criticalangle of the dielectric, an almost uniform amplitude distribution isobtained across the lens (FIGURE 4). In addition, the transfer of energyfrom exciter 12 to the base of the dielectric structure is accomplishedin an extremely efficient manner with an unusually small amount ofenergy lost in spillover past the lens, resulting in a high directivityfor the resultant radiation pattern.

For purposes of explaining the theory of operation of antenna systems inaccordance with the present invention, resort is had to the heuristicray tracing convention. Briefly, this convention consists of drawingvectors perpendicular to surfaces of equal phase and applying to thevectors or rays the optical laws governing light rays. The technique isespecially appropriate where antenna dimensions are large in terms ofwave lengths of the electromagnetic energy involved. Reference will alsobe made to the radiation pattern and resultant amplitude distributionillustrated in FIGURES 3 and 4, respectively. As shown in FIGURE 2,those rays striking the guiding structure boundary at an angle greaterthan the critical angle 3 of the guide dielectric material are totallyreflected toward, and are thus incident on the lens. As is well known,

,=arc sin 1/\/s, where e is the dielectric constant of the guide. Suchrays would, in the absence of the guiding structure, fail to strike thelens and would represent energy loss in the form of spillover lobes. Theguiding structure flare -or taper angle is selected to be on the orderof the complement of the critical angle, i.e. -90 and theelectromagnetic transducer is designed to provide a first null of theradiation pattern at an angle equal to the sum +(90 to provide an almostuniform amplitude distribution (FIGURE 4) across the lens with verylittle loss caused by spillover radiation. The result is an apertureefficiency for the antenna system approaching 100 percent.

The darkened portion of the exciter pattern in FIG- URE 3 representsthat portion of the energy radiated by the exciter which would normallybe lost in spillover lobes in the absence of the' guiding structure. Asillustrated in FIGURE 3, the first null of the radiation pattern occursat approximately an angle of twice the flare angle of the structure.

The amplitude distribution at the base of the dielectric guidingstructure as illustrated in FIGURE 4 is the result of the summation ofwaves incident on the lens in a direct path from the exciter and wavesreflected from the boundary of the guiding structure toward the lens.

In practice, any lens may be employed which is capable of converting thespherical wavefront emanating from the exciter to a plane wavefront toproduce the desired highly directive antenna system. In one antenna.

system, built in accordance with principles of the present invention asdiscussed herein, the dielectric guiding structure was a solid conehaving a dielectric constant e of 1.02, a base diameter of approximately18 inches, and a flare or taper angle of approximately 6.5 degrees whichwas substantially the complement of the structure dielectric criticalangle. The antenna system yielded a measured gain of 33 db at 9.4 gc.,corresponding to percent aperture efliciency. In a second practicalembodiment, the dielectric guiding structure again 'had a dielectricconstant of 1.02, a base diameter of approximately 18.5 inches and aflare or taper angle approximately equal to twice the complement of thecritical angle or about 15 degrees. In the latter system, the measuredgain was 30.8 db at 8 gc. corresponding to about 77 percent apertureefficiency. In other embodiments, the taper angle of the dielectricstructure was varied between one-third and three times the criticalangle of the dielectric with suitable results.

Wave polarization is not critical in antenna systems according to thepresent invention, and may thus be of any conventional form, forexample, linear, circular, and so forth, as dictated by the exigenciesof the specific use or application.

While I have described one particular embodiment of my invention, itwill be understood that various changes and modifications in thespecific details of construction and operation described may be resortedto without departing from the true spirit and scope of the invention asdefined by the appended claims.

I claim:

1. In an antenna system for translating electromagnetic wave energybetween a signal line and free space, an electromagnetic wave transducercoupled to said transmission line, lens means for refraction and phasecorrection of said electromagnetic wave energy, said lens means having afocal point in proximity to the phase center of said transducer, anddielectric guiding means for establishing a boundary between thedielectric medium thereof and free space to substantially confine thepropagation of electromagnetic waves between said transducer and saidlens means interiorly of said boundary, wherein said dielectric guidingmeans comprises a solid dielectric mass having a monotonicallyincreasing cross-sectional area from said transducer to said lens, saidguiding means having an angle of taper at least approximately equal tothe complement of the critical angle of said boundary, where saidcritical angle is the angle of incidence of said electromagnetic waveson said boundary above which total reflection of the incident waveobtains.

2. A dielectric antenna comprising a source of electromagneticradiation, a lens for correction of the phase distribution of and forrefraction of electromagnetic waves incident thereon from said source,and dielectric wave guide means interposed between said source and saidlens for directing electromagnetic waves emanating from said sourcetoward said lens and for inhibiting the escape of electromagnetic waveenergy into free space from the region between said source and saidlens;

wherein said dielectric waveguide means comprises a solid dielectricstructure having a dielectric constant relative to the dielectricconstant of free space defining a critical angle at the boundarytherebetween such that electromagnetic waves incident on said boundaryfrom within said structure at an angle greater than said critical angleare reflected therefrom to strike said lens;

wherien said dielectric structure has a conical configuration, said lensbeing disposed at the larger diameter end of said structure and having afocal point within said structure corresponding to the phase center ofsaid source, said conical configuration having an angle of taper ofbetween approximately one-third to three times the complement of thecritical angle of said boundary.

3. A dielectric antenna comprising a source of electromagneticradiation, a lens for correction of the phase distribution of and forrefraction of electromagnetic Waves incident thereon from said source,and dielectric Waveguide means interposed between said source and saidlens for directing electromagnetic waves emanating from said sourcetoward said lens and for inhibiting the escape of electromagnetic waveenergy into free space from the region between said source and saidlens,

wherein said dielectric guide means comprises a solid dielectricstructure having a dielectric constant relative to the dielectricconstant of free space defining a critical angle at the boundarytherebetween such that electromagnetic waves incident on said boundaryfrom within said structure at an angle greater than said critical angleare reflected therefrom to strike said lens,

wherein said dielectric structure has a conical configuration, said lensbeing disposed at the larger diameter end of said structure and having afocal point Within said structure corresponding to the phase center ofsaid source, said conical configuration having an angle of taper ofbetween approximately onethird to three times the complement of thecritical angle of said boundary,

wherein said lens comprises a solid dielectric medium having a convexsurface upon which said electromagnetic waves emanating from said sourceare incident, and having a dielectric constant greater than saiddielectric constant of said dielectric structure for convert-ing aspherical wave front to a plane wave front.'

4. In an antenna system for translating electromagnetic wave energybetween the signal transmission line and free space:

source means coupled to said transmission line for directingelectromagnetic wave energy in a given direction and in a radiationpattern having at least a first radiation null at a predetermined anglerelative to said given direction;

lens means for correcting the phase distribution of and for retractingthe electromagnetic wave energy generated by said source means; and

' solely dielectric waveguide means interposed between said source meansand said lens means for interiorly confining all of the electromagneticwave energy generated by said source within said predetermined angle,said dielectric waveguide means comprising a dielectric mass having amonotonically increasing cross-sectional area in proceeding from saidsource means to said lens means, and having an angle of taperapproximately equal to the complement of the critical angle at theboundary of said dielectric waveguide means,

said critical angle being the angle of incidence of internalelectromagnetic waves on said boundary above which total reflection ofthe incident wave occurs.

5. In an antenna system for translating electromagnetic wave energybetween the signal transmission line and free space:

source means coupled to said transmission line for directingelectromagnetic wave energy in a given direc- 5 tion and in a radiationpattern having at least a first radiation null at a predetermined anglerelative to said given direction;

lens means for correcting the phase distribution of and for refractingthe electromagnetic wave energy generated by said source means; and vsolely dielectric Waveguide means interposed between said source meansand said lens means for interiorly confining all of the electromagneticwave energy generated by said source within said predetermined angle,said dielectric waveguide means comprising a dielectric mass having aconical configuration with a monotonically increasing cross-sectionalarea' in proceeding from said source means to said lens means, andhaving an angle of taper approximately equal to the complement of thecritical angle atthe boundary'of said dielectric waveguide means,

said critical angle being the angle of incidence of internalelectromagnetic waves on said boundary above which total reflection ofthe incident wave occurs,

said lens means having a focal point within said dielectric masscorresponding to the phase center of said source means.

6. In combination:

an electromagnetic wave launcher,

lens means positioned relative to said launcher to modify the phasedistribution of electromagnetic waves supplied thereto from saidlauncher,

a tapered solid dielectric waveguide extending in in= creasing girthfrom said electromagnetic wave launcher to said lens and coupled to saidwave launcher to receive electromagnetic wave energy therefrom saidwaveguide having a dielectric constant substantially greater than one,and having an angle of taper approximately equal to the complement ofthe critical angle for totalinternal reflection of electromagnetic wavesemanating from said launcher from the boundary formed by the taperedsurface of said waveguide.

7. The combination according to claim 6 wherein said launcher has aphase center at the focal point of said lens means.

.' References Cited UNITED STATES PATENTS 2,596,190 5/1952 Wiley 343 7s32,822,541 2/1958 Sichak e161 343-786X 3,005,983 10/1961 Chandler943-5753 3,321,763 5/1967 1mm et al. 343454 FOREIGN PATENTS 9/1953Germany.

4/ 1960 Germany. 11/1963 Canada.

ELI LIEBERMAN, Primary Examiner,

